Briefly, this comes from one of Socrates’ dialogs, in which he asks a man named Euthyphro whether certain things are good because the gods say so, or whether the gods say those things are good because they are good. This also applies to the Christian god, naturally.
In the first case, morality is inherently arbitrary and subjective: if God decides that rape is good, then rape automatically becomes good; but this feels wrong, aside from being abhorrent.
The second case is that certain actions are good on their own merits: that charity is good and rape are bad for reasons that have nothing to do with any gods. But in that case, the gods are irrelevant to morality.
Now, personally, I don’t see a problem with the second one. If a god showed up and told me to do or not do something, I’d want him/her/it to explain why, and to provide a better explanation than “Meh. I flipped a coin, and today, pork is tref”.
But of course Hanegraaff sees his god as the source of morality, and not merely a middleman or a teacher. So the way he gets around the dilemma is to say that yes, things are good because God says they are, but also God wouldn’t command something like rape to be moral, because such is not his nature: things are good because they reflect God’s nature.
The problem is that this doesn’t solve the problem: it merely redefines “good” to mean “like God”, with no connection to anything else, like happiness or well-being, or anything like that. Is killing someone good? It is, if it reflects God’s nature. So how can we tell whether it reflects God’s nature? Given the number of people that Yahweh kills in the Bible, it seems that killing, and even genocide, reflects his nature.
I expect Hanegraaff would dispute this, saying, for instance, that it was okay for God to kill everyone in the flood because they deserved it, or that slavery was necessary back in ancient Israel, or It’s Okay When God Does It, or whatever. But that’s just it: why is it even necessary to explain why the Bible seems to describe God as doing horrible things? Especially if the Bible is a true account of what happened, and especially if, as many apologists say, God’s law is written on all our hearts? Wouldn’t we read about the massacre of the Midianites and think, “Yeah, they had it coming”? Would anyone balk at the image of Jesus whipping people with a scourge he made himself?
It seems clear that even if “good” and “evil” aren’t clearly defined and have fuzzy edges, Hanegraaff isn’t relying solely on the Bible to figure out what falls in each category, and neither does anyone else. That is, whatever “good” means, it’s not synonymous with “like God”.